1. Field of the Invention
The present invention relates to an electrochemical generator apparatus containing solid oxide electrochemical cells containing fuel electrodes operating in a hydrocarbon fuel environment, where the fuel electrodes are impregnated with selected chemicals to form metal oxides upon heating. The metal oxides prevent cell degradation which was caused by carbon formation.
2. Description of the Prior Art
High temperature solid oxide fuel cells convert chemical energy into electrical energy, typically at temperatures of from about 800.degree. C. to 1,200.degree. C. Such solid oxide fuel cells, solid oxide fuel cell configurations, and solid oxide fuel cell generators are well known, and taught, for example, by Isenberg in U.S. Pat. Nos. 4,490,444 and 4,664,987, Makiel in U.S. Pat. No. 4,640,875, and Somers et al., in U.S. Pat. No. 4,374,184. In all of these patents, an axially elongated, tubular air cathode, usually made of a doped oxide of the perovskite family, has solid oxide electrolyte deposited over it, except for a small radial segment which contains a deposit of interconnection material down the length of the tube. A fuel anode, usually a nickel-zirconia cermet, forms an outer layer over the electrolyte, to provide a fuel cell. A metal fiber, current collector felt is then attached to the interconnection. Other fuel cell configurations are also known, such as those taught by Isenberg, in U.S. Pat. No. 4,728,584.
Typical fuel electrode construction for tubular solid oxide fuel cells is taught by Isenberg et al. in U.S. Pat. No. 4,582,766 and Isenberg, in U.S. Pat. No. 4,702,971, the latter of which contains a ceria or urania ionic-electronic conductive coating to increase sulfur tolerance of the electrode. In all of these applications, the cells and cell assemblies can be electrically connected in series and parallel, hydrocarbons or other fuel can be fed to contact the fuel electrode on the exterior surface of the fuel cell and air or oxygen can be fed to contact the central, interior air electrode. Insulation used with all of these cell designs is usually low density alumina.
Utilization of both methane, and natural gas containing higher hydrocarbons as fuels is possible in the axially elongated interconnection designed cells. However, there is a possibility of some performance degradation and electrical shorting between the fuel electrode and the interconnection due to carbon deposition, especially when higher hydrocarbons are used in the fuel. Also, in some instances, during cell operation, fuel electrodes are prone to deactivation for the reformation of methane and other hydrocarbon fuels along at least part of the cell length.
Carbon deposition on the fuel electrode surface is thought to result from poor water adsorption on the fuel electrode, leading to slower gasification of carbon from adsorbed hydrocarbons at the surface. If H.sub.2 O is not adsorbed, the adsorbed oxygen species necessary to react with adsorbed carbon species, to form CO and CO.sub.2 gases, will not be present in sufficient quantity, and will result in formation of carbon which is encapsulating in nature and remains resistant to oxidation, even though H.sub.2 O is present in the fuel atmosphere.
Removal of such carbon, once formed at electrode surfaces, is usually very difficult. Formation of a continuous layer of carbon between the fuel electrode and the interconnection may also, in some circumstances, after prolonged use, result in the development of an electrical short circuit path and cause performance decay of the cell. Formation of a surface layer of carbon on the fuel electrode prevents fuel species from reaching the electrochemical reaction zone at the electrolyte interface, thus degrading fuel cell performance.
In the area of catalytic reforming of heavy gaseous and/or liquid hydrocarbons containing sulfur, utilizing the injection of steam to produce hydrogen, but not involving fuel cells, Setzer et al., in U.S. Pat. No. 4,451,578, teaches high activity iron oxide catalysts which demonstrate a better resistance to carbon plugging than nickel catalysts. The catalyst can be unsupported and contain 90% FeO or Fe.sub.2 O.sub.3 plus modifiers such as Al.sub.2 O.sub.3, K.sub.2 O, CaO or SiO.sub.2, or the catalyst can be unmodified, and supported on Al.sub.2 O.sub.3, CaO impregnated Al.sub.2 O.sub.3 and La stabilized Al.sub.2 O.sub.3. In a typical example, O.318 cm (0.125 inch) diameter Al.sub.2 O.sub.3 pellets were impregnated with Ca(NO.sub.3).sub.2, placed in an ultrasonic blender, dried, and then calcined at 1010.degree. C. This material was then impregnated with Fe(NO.sub.3).sub.3.9H.sub.2 O, dried, and then calcined at 1000.degree. C.
What is needed, for fuel cells using a hydrocarbon fuel, is a means to prevent carbon formation on the anode electrode exposed electrolyte, and metal fiber current collector felts, in a fuel cell or cell bundle operating at 800.degree. C. to 1200.degree. C. with a hydrocarbon fuel feed. The main object of this invention is to provide such a means.